This invention relates to fibre channel arbitrated loop network architecture and more particularly to an adaptation of fibre channel architecture to switchable star architectures. The purpose of the invention is to increase the throughput of fibre channel arbitrated loop networks.
Fibre channel networks are a particular class of high-speed networks defined by ANSI standard X3T11. Fibre channel networks can be constructed in three different topologies: point-to-point, arbitrated loop and fabric switch. Point-to-point networks consist of two stations connected directly to (and only to) each other. Fabric switches can connect networks of up to 16 million stations and provide multiple classes of service (connection-oriented, connection-less with acknowledgement, connection-less without acknowledgement, or intermix).
Referring to FIG. 1, an arbitrated loop topology 10 is shown, which is a shared-bandwidth, logical ring topology designed for low-cost attachment of such stations 12 as servers and disk array devices. Up to 126 stations can communicate on an arbitrated loop 14, but only two stations can communicate interactively at any one time (with an exception for broadcast or multicast conversations). The arbitrated loop standard also allows for a single fabric port 16 to be resident on the loop for connection into the larger network.
In an arbitrated loop topology, stations 12, 16, 18, 20, 22 that intend to communicate on the loop 14 must "arbitrate" for access. Stations signal their intent to arbitrate by issuing a primitive signal for circulation around the loop to all nodes or stations, called an ARB. The ARB is a special ordered set signal which contains the identity of the station that requests access. When multiple stations request access simultaneously, the station with the lowest physical address prevails. (Physical addresses are assigned cooperatively by the stations each and every time the loop topology changes.) A station 12 that wins arbitration then "opens" the station 18 with which it wants to communicate by sending an OPN primitive, which is a special ordered set signal including the address of the target station as an argument. The two stations 12, 18 then communicate in either half-duplex or full-duplex fashion, until both stations agree that the conversation is finished.
Arbitrated loops are typically wired together using hub devices 15 in which the loop is formed among the connected stations. A hub device 15 allows multiple stations to be connected together, although only one conversation can take place at any instant in time. This is analogous to networking topologies such as Token Ring (ISO/IEEE 802.5) and Ethernet (ISO/IEEE 802.3).
Referring to FIG. 2, in networking topologies, such as loop networks 014, 114, 214, 314, the conventional prior art approach to increasing the available bandwidth is to provide passthrough switches 24, 26, 28 which allow multiple concurrent conversations to take place. The disadvantage is a need for extra switching logic and buffer memory to support connections. In fibre channel technology it is conventional to use the fabric switch topology when greater bandwidth is needed. However, fabric switches tend to be complex and expensive, particularly given that they must also implement a large number of standardized features, such as address space resolution, and support for multiple classes of service.
Any modification to the arbitrated loop must behave such that devices connected to it are unaware that they are not on a normal loop when they are communicating. This is important for interoperability with any existing fibre channel arbitrated loop hardware on the market. What is needed is a mechanism whereby any switching element presents an interface equivalent to a loop.